Reduce build output to avoid maximum log length problems on CI

It's not too uncommon that as we upgrade artifacts used by our build that means that transitive new dependencies get included or that the new artifacts do more logging than before resulting it the CI builds failing due to the maximum log length being hit. One of the biggest offenders and probably the least needed aspect of our CI logs comes from Maven itself logging the downloading of all the artifacts needed for each build step. We had already converted the build to using batch-mode and that worked for a while but it does not seem to be enough, particularly on connectors that are more verbose than others (looking at MongoDB).

I propose that we add one additional command line argument to the maven steps in Travis:

-Dorg.slf4j.simpleLogger.log.org.apache.maven.cli.transfer.Slf4jMavenTransferListener=warn

This suppresses all the "Downloading" and "Downloaded" log lines in batch mode so they are no longer emitted in the logs, which will reduce the size of the CI logs pretty substantially.